1. Field of the Invention
This invention relates to gas turbine engines and more particularly to apparatus for cooling the walls of a vane which is disposed across the path of working medium gases in the turbine section of the engine.
2. Description of the Prior Art
A limiting factor in many turbine engine designs is the maximum temperature of the working medium gases which can be tolerated in the turbine without adversely limiting the durability of the individual components. The rotor blades and the nozzle guide vanes of the turbine are particularly susceptible to thermal damage and a variety of cooling techniques is applied to control the temperature of the material comprising these components in the face of high turbine inlet temperatures. In many of these techniques air is bled from the compressor to the local area to be cooled through suitable conduit means. Compressor air is sufficiently high in pressure to cause the air to flow into the local area of the turbine without auxiliary pumping and is sufficiently low in temperature to provide the required cooling capacity.
Most recently considerable design effort has been expended to minimize the amount of air consumed for cooling of the turbine components. Impingement cooling is one of the more effective techniques and occurs where a high velocity air stream is directed against a component to be cooled. The high velocity stream impinges upon a surface of the component and increases the rate of heat transfer between the component and the cooling air. A typical application of impingement cooling is discussed by Smuland et al in U.S. Pat. No. 3,628,880 entitled "Vane Assembly and Temperature Control Arrangement". Smuland et al shows baffle plates interposed between the cooling air supply and the surface to be cooled. Orifices in each plate direct jets of the cooling air across an intermediate space between the baffle and the cooled surface during operation of the engine. The pressure ratio across each plate is sufficiently high to cause the cooling air to accelerate to velocities at which the flow impinges upon the opposing surface. Cooling air is exhausted from the intermediate space between the plate and the opposing surface at a high rate to prevent the buildup of backpressure within the space. In Smuland et al film cooling passageways are utilized to exhaust the impingement flow.
A second highly effective but not as widely utilized technique is that of transpiration cooling. A cooling medium is allowed to exude at low velocities through a multiplicity of tiny orifices in the wall of the component to be cooled. The low velocity flow adheres to the external surface of the component and is carried axially downstream along the surface by the working medium gases flowing thereacross. In transpiration cooling the exuding velocities must remain low in order to prevent over penetration of the working medium gases by the cooling air. Over penetration interrupts both the flow of cooling air and the flow of medium gases and renders the cooling ineffective. One typical application of transpiration cooling to a turbine vane is discussed by Moskowitz et al in U.S. Pat. No. 3,706,506 entitled "Transpiration Cooled Turbine Blade with Metered Coolant Flow". Moskowitz et al shows a plurality of coolant channels formed across the chord of the blade to accommodate both temperature and pressure gradients across the chord. Cooling air is flowed to each channel through a metering plate at the base of the airfoil section. A preferred pressure ratio across the cooled wall in most transpiration cooled embodiments is approximately (1.25). The effectiveness of a transpiration cooled construction is highly sensitive to variations from the designed pressure ratio across the surface to be cooled; accordingly, the pressure ratio must be closely controlled.
Impingement and transpiration cooling are combined in one airfoil section in U.S. Pat. No. 3,726,604 to Helms et al entitled "Cooled Jet Flap Vane". The impingement cooling is applied to the leading edge of the airfoil and the transpiration cooling is applied to the suction and pressure walls, however, both cooling techniques are not applied simultaneously to supplement each other in cooling a common portion of the vane wall.
The platform region at the base of each guide vane is also cooled in many constructions. In U.S. Pat. No. 3,610,769 to Schwedland et al, cooling air is flowed in accordance with transpiration cooling techniques into the working medium flow path at low velocities through small diameter cooling holes in the platform of each vane. In FIG. 1 of Schwedland et al it is apparent that the entire platform of each vane is fed with cooling air from a single supply chamber which extends beneath the suction and pressure sides of the platform and over the entire axial length of the platform.
The above described cooling techniques, have been successful in prolonging the life of various turbine components, however, the requirement for even more durable, high performance engines exists. More effective ways of cooling with smaller quantities of air than are presently required are being sought.